Method for control of a gearbox

ABSTRACT

A method for control of a gearbox, installed in a motor vehicle ( 1 ): The method effects a downshift of the gearbox ( 20 ) from a first gear (G 1 ), for which the acceleration α of the vehicle ( 1 ) is negative, to a second gear (G 2 ), for which the acceleration α is positive or substantially equal to nil. The downshift involves at least one intermediate gear step between the first gear (G 1 ) and the second gear (G 2 ), using an engine speed ω G1  in the first gear (G 1 ) as an input parameter when effecting the downshift. Also a system, a motor vehicle, a computer program and a computer program product for performing the method are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a 35 U.S.C. §§371 national phase conversionof PCT/SE2010/050980, filed Sep. 14, 2010, which claims priority ofSwedish Application No. 0901182-6, filed Sep. 14, 2009, the contents ofwhich are incorporated by reference herein. The PCT InternationalApplication was published in the English language.

TECHNICAL FIELD

The present invention relates to a method for control of a gearbox. Theinvention further relates to a system, a motor vehicle, a computerprogram and a computer program product for performing the method.

BACKGROUND TO THE INVENTION

FIG. 1 depicts schematically parts of a power train for a motor vehicle1, such as a passenger car or a heavy vehicle 1, e.g. a truck or bus.The power train comprises an engine 10 mechanically connected by a shaftto a first end of a gearbox 20 via a clutch device 40. The gearbox 20 isalso mechanically connected, at its other end, by a propeller shaft 50to a differential gear 30 associated with a rear axle. The rear axlecomprises respective left and right drive shafts 60 which drive thevehicle's 1 powered wheels (not depicted in the diagram).

With this well-known arrangement, the mechanical work of the engine 10is transmitted via various transmission devices, e.g. clutch device 40,gearbox 20, propeller shaft 50, differential gear 30 and drive shafts60, to powered wheels in order to move the vehicle 1. An importantdevice in the power train is the gearbox 20, which has a number offorward gears for moving the vehicle 1 forwards, and usually also one ormore reverse gears. The number of forward gears varies but modern kindsof trucks are, for example, usually provided with twelve forward gears.

The gearbox 20 may be of manual or automatic type (automatic gearbox),but also of the automatic manual gearbox type (automatic manualtransmission, AMT). Automatic gearboxes and automatic manual gearboxesare automated gearbox systems usually controlled by a control unit 110,sometimes also called electronic control unit (ECU), which is adapted tocontrolling the gearbox 20, e.g. during gear changing for choice of gearat a certain vehicle speed with a certain running resistance. The ECUmay measure the speed of the engine 10 and the state of the gearbox 20and control the gearbox 20 by means of solenoid valves connected tocompressed air devices. Information about the engine 10, e.g. its speedand torque, is also sent from the engine 10 to the ECU, e.g. via a CAN(controller area network) bus in the vehicle 1.

In conventional gear change systems, the control unit 110 uses tabulatedengine speed limits, also called shift points, which represent theengine speed at which a downshift or upshift should be effected in thegearbox 20. This means that the system changes gear when the speed ofthe engine 10 passes a speed represented by a shift point. The shiftpoints may therefore be construed as providing information not onlyabout when a downshift or upshift should take place but also about thenumber of gear steps to be effected at each downshift or upshift. It isusual for each shift point to comprise one to three gear steps, althoughmore steps are possible.

FIG. 2 depicts an example of various tabulated shift points representedby lines SP1-SP6 in a graph where the x axis represents engine torqueand the y axis the speed of the engine 10 in revolutions per minute(rpm). So long as a current engine speed is between shift lines SP1 andSP4 no gear change takes place, but if the current engine speed passesan upshift line, SP1-SP3, an upshift is initiated, and conversely adownshift is initiated if the current engine speed drops below adownshift line, SP4-SP6.

Table 1 below shows a number of upward or downward gear steps for eachof the lines SP1-SP6 in FIG. 2. For example, an upshift by one steptakes place if the engine speed rises above line SP1 and a downshift bytwo steps take place if the engine speed drops below line SP5.

TABLE 1 Numbers of gear steps for downshift and upshift lines SP1-SP6SP1 Engine speed for upshift by 1 step SP2 Engine speed for upshift by 2steps SP3 Engine speed for upshift by 3 steps SP4 Engine speed fordownshift by 1 step SP5 Engine speed for downshift by 2 steps SP6 Enginespeed for downshift by 3 steps

Shift point choices affect inter alia running characteristics,acceleration, comfort and fuel consumption for the vehicle 1, so shiftpoints have to be accurately calibrated by vehicle manufacturers. Thiscalibration generally involves various gear change strategies beingtested in the field in different driving situations, e.g. with differentamounts of acceleration applied, different road gradients and differentvehicle-combination weights. The test results have then to be thoroughlyanalysed to determine appropriate shift points, which is verytime-consuming since there is an almost infinite number of combinationsof different power trains, driving situations and vehicle weights.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to propose a method for control of agearbox which wholly or partly solves the problems of the state of theart. Another object of the present invention is to propose analternative method for control of a gearbox.

According to an aspect of the invention, the above objects are achievedwith a method for control of a gearbox intended to be installed in amotor vehicle, which method effects a downshift of said gearbox from afirst gear for which the acceleration α of said vehicle is negative to asecond gear for which the acceleration α is positive or substantiallyequal to nil, which downshift involves at least one intermediate gearstep between said first and second gears, using an engine speed ω_(G1)in said first gear (G1) as an input parameter when effecting saiddownshift.

The invention relates also to a computer program which comprises programcode and which, when said program code is executed in a computer, causessaid computer to effect the above method for control of a gearbox. Theinvention relates also to a computer program product belonging to saidcomputer program.

According to another aspect of the invention, the above objects areachieved with a system for control of a gearbox, which system comprisesat least one control unit intended to control a gearbox in a motorvehicle, which system is adapted to effecting a downshift of saidgearbox from a first gear for which the acceleration α of said vehicleis negative to a second gear for which the acceleration α is positive orequal to nil, which downshift involves at least one intermediate gearstep between said first and second gears, using an engine speed ω_(G1)in said first gear (G1) as an input parameter when effecting saiddownshift.

The system according to the invention may also be modified in accordancewith the various embodiments of the above method. The invention relatesalso to a motor vehicle comprising at least one system as above.

An advantage of the present invention is that since a downshiftaccording to the invention is based on an engine speed which the vehicle1 had when it began the climb, i.e. the engine speed which the vehiclehad when it went into power deficit ω_(G1), the shift points areautomatically adjusted to achieve the same behaviour for different powertrains. For this reason, calibration of shift points is not related todifferent engine types or power trains but only to different drivingmodes, e.g. economy mode or power mode. The advantage of this procedureis that time need not be involved in calibration with respect todifferent types of power trains or engines 10, making it possible toconcentrate instead on calibration of a general behaviour for every typeof vehicle 1, thereby saving time and reducing costs involved incalibration.

Another advantage of the invention is that if a driver him/herselfchanges down manually before or on a hill, the gear change systemaccording to the invention will treat the engine speed increased by thedriver in the same way as if the system had itself done an aggressivedownshift. The system will accordingly continue to change gearaggressively all the way up the hill. This makes it easy for the driverto control the way the hill climb is conducted, since a high enginespeed value for the first gear G1 ω_(G1) leads to more aggressive hillclimbing than a lower engine speed value for the first gear G1 ω_(G1).

Further advantages and applications of a method and a system accordingto the invention are indicated by the detailed description set outbelow.

BRIEF DESCRIPTION OF DRAWINGS

The present invention is described with reference to the attacheddrawings, in which:

FIG. 1 depicts schematically part of a power train for a motor vehicle;

FIG. 2 is a graph of downshift and upshift lines;

FIG. 3 is a flowchart of an embodiment of the invention and consists ofFIGS. 3A and 3B;

FIG. 4 depicts an example of downshift from a first gear G1 to a secondgear G2 according to the invention; and

FIG. 5 depicts a control unit forming part of a system according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Conventional gear change systems described above choose gears accordingto prevailing running conditions, using fixed shift points such asdepicted in FIG. 2. If the vehicle 1 is for example embarking upon anupgrade and a current gear is not appropriate because the vehicle 1 islosing speed uphill, the gear change system has to choose a differentgear for the vehicle 1 to run in.

A problem in this situation is that of choosing a gear which results inlow fuel consumption but also causes the engine 10 to run at a speedwhich delivers sufficient power output for the driver to feel that thevehicle 1 is powerful all the way up the hill. Powerful means here thatthe speed of the engine 10 is close to its maximum power output speed.

As vehicles 1 may have different specifications, e.g. gearbox ratio,rear axle ratio and wheel radius, the same vehicle speed may be achievedat different engine speeds for vehicles 1 with different specifications.This makes the use of fixed shift points problematical in that they maysuit certain vehicle specifications but not others. The same kind ofproblem may also arise if for example a vehicle 1 changes from one wheelradius to another, resulting in a different overall transmission ratio.

A situation which may therefore arise when a vehicle 1 is travellinguphill is that a downshift may take place too early or too late becausethe fixed shift points are not appropriate to a certain vehiclespecification. Too early a downshift may make the vehicle 1 feel“nervous”, i.e. changing gear too often and being unstable, along withincreased fuel consumption. Too late a downshift means that the poweroutput of the engine 10 is not utilised in an optimum way, since thevehicle 1 loses more speed than necessary.

The present invention therefore relates to a method which implements agear change strategy for a gearbox 20 and which wholly or partlyeliminates the disadvantages of the state of the art. The gearbox 20 ispreferably of the kind which forms part of an automated gear changesystem controlled by a control unit 110 (ECU). In such a system, gearchanges are effected automatically by the control unit 110, but it isalso usual for the driver to be able to execute manual gear changes insuch an automated gear change system, this being known as manual gearchange in automatic state (automatic mode). The gearbox 20 alsocomprises a plurality of gears, e.g. 12 forward gears and one or morereverse gears.

The concept with a method according to the invention is that a downshiftstrategy is applied whereby the gear change system changes from a firstgear G1 for which the acceleration α of the vehicle 1 is negative (powerdeficit gear) to a second lower gear G2 for which the vehicle's 1acceleration α is positive or equal to nil, via one or more intermediategear steps. According to the present invention an engine speed ω_(G1) insaid first gear (G1) is also used as as an input parameter wheneffecting said downshift. This speed ω_(G1), is a speed which the engine10 has when the vehicle 1 is driven with the first gear G1 engaged.

According to an embodiment of the invention, said engine speed ω_(G1) ispreferably the speed which the vehicle 1 has when it goes into a stateof power deficit in the first gear G1, e.g. when embarking upon anupgrade. This means that, according to another embodiment of theinvention, the highest engine speed in the first gear G1 will be saidspeed ω_(G1), and this may be construed as an engine speed at which arelevant driving force in the first gear G1 becomes smaller than arelevant running resistance in the same gear. FIG. 4 depicts an exampleof said engine speed ω_(G1).

The expressions “negative” and “positive or equal to nil” in thisdescription are to be taken to mean substantially “negative” andsubstantially “positive or equal to nil” respectively. The reason isthat the acceleration α of the vehicle 1 may momentarily assume a valuewhich is “positive or equal to nil” for the first gear G1, but theacceleration α averaged over a period of time may nevertheless benegative. As specialists will appreciate, the same also applies withregard to the vehicle's acceleration α when travelling with the secondgear G2 engaged.

“Power deficit gear” in this description means a gear with a ratio suchthat the vehicle 1 does not have enough driving power to maintain aconstant speed in that gear. “Power equilibrium gear” means the highestgear in which the vehicle 1 can maintain a constant speed, i.e. thevehicle 1 being in power equilibrium. It should be noted that thenegative acceleration α in the first gear G1 is due to the engine 10 notbeing able to deliver sufficient power in the first gear G1, resultingin power deficit.

According to an embodiment of the invention, a highest engine speed ateach intermediate step is as high as, or higher than, a highest enginespeed at a preceding intermediate step. The engine speed for eachintermediate step is for example within the range 1000-2500 rpm fortrucks and buses.

According to a further embodiment of the invention, a highest enginespeed at each intermediate step is as high as, or higher than, a highestengine speed at a preceding intermediate step.

According to another embodiment of the invention, the highest enginespeed at each intermediate gear step increases by a parameter valuep_(i). The relationship between the highest engine speed at anintermediate step and the highest engine speed at the precedingintermediate step is preferably ω_(T) ^(i+1)=ω_(T) ^(i)+p_(i), whereω_(T) ^(i+1) is the highest engine speed at an intermediate step andω_(T) ^(i) is the highest engine speed at the preceding intermediatestep.

The engine speed ω_(T) ^(i) at an intermediate gear step is thus ahighest engine speed which the vehicle 1 has had since the accelerationα became negative, i.e. the highest engine speed which the vehicle 1 hashad since it went into power deficit. Said engine speed ω_(T) ^(i) maybe construed as a target speed which the gear change system endeavoursto reach after each intermediate step in the downshift according to thisembodiment of the invention. The parameter value p_(i) may also beconstrued as a tolerance value, since the engine speed after each gearchange may deviate from a simulated speed. The magnitude of thetolerance value p, may also be varied to influence how quickly saidengine speed ω_(T) ^(i) should increase, e.g. for different drivingmodes such as economy mode or power mode. It should also be noted thatthe parameter value p_(i) need not be constant but may be variable andassume different values for different gears.

According to a further embodiment of the invention, the acceleration αis nil or substantially nil for the second gear G2. The acceleration α,expressed for example in m/s² or rpm/s, for the second gear G2 may becompared with a threshold value A to check whether the condition ofbeing positive or nil is fulfilled. The acceleration α may also becompared with this threshold value A during a specific period of time toavoid momentary deviations of the acceleration α which might lead toincorrectness in the comparison. Checking the value of the accelerationα for the second gear may also be done by comparing a calculated runningresistance with a calculated driving force to decide whether theacceleration α will be greater than or equal to nil in the second gearG2.

With regard to intermediate gear steps in the present method it ispossible for one or more of them to be effected between the first gearG1 and the second gear G2 according to an embodiment of the invention.Moreover, the number of steps between the first gear G1 and anintermediate step, and/or between two consecutive intermediate steps,and/or between an intermediate step and the second gear G2, may be oneor more.

According to a preferred embodiment of the invention, the time whicheach intermediate gear step lasts is longer than a threshold valueT_(i). How long the vehicle 1 can run in a certain intermediate gear maypreferably be compared with a gear-specific calibrated threshold valueT_(i). This means that, according to an embodiment of the invention, ifan intermediate gear is to be regarded as a permissible intermediatebetween the first gear G1 and the second gear G2, the vehicle 1 has tobe able to run in it for at least a longer time than the threshold valueT_(i) for that specific gear.

The reason why it is not appropriate for the vehicle 1 to run for tooshort a time in an intermediate gear is that when the vehicle changesgear it loses power transmission from the power train and will thereforelose speed during the actual gear change process. For this reason it ispreferable for such a situation to be avoided, since the vehicle 1 maylose more speed if the gear change system chooses to change gear via anintermediate gear than if it skips said intermediate gear and insteadchanges down a further one or more steps during the downshift.

Effecting a downshift and thereafter running in the resulting gear fortoo short a time also entails discomfort for the driver and anypassengers, so the gear change system may use gear-specific calibratedthreshold values T_(i) as described above. These calibrated thresholdvalues T_(i) also determine how likely it is that the system will orwill not skip an intermediate gear when calculating a downshift from thefirst gear G1 to the second gear G2. The greater the value adopted as agear-specific calibrated threshold value T_(i), the more likely it isthat the system will skip an intermediate gear, whereas the smaller thevalue adopted as the threshold value T_(i), the less likely it is thatthe system will skip an intermediate gear. Accordingly, the thresholdvalue T_(i) may be used as a parameter for determining the number ofgear steps at each intermediate step and/or for determining the numberof intermediate steps between the first gear G1 and the second gear G2according to different embodiments of the invention.

Time values for the gear-specific threshold values T_(i) may preferablyassume a value of between 1 and 15 seconds for heavy vehicles 1, e.g.trucks and buses, depending on the behaviour desired during thedownshift, e.g. downshift rate and downshift rhythm. The thresholdvalues T_(i) may therefore be used as design parameters in theconfiguration of different downshift behaviours of the gear changesystem, since the threshold values T_(i) will determine the number ofintermediate steps and the number of gear steps at each intermediatestep as described above.

According to another embodiment of the invention, a current intermediategear step lasts the same amount of time as an immediately followingintermediate step, since the downshift will then be felt to beconsistent and positive by most drivers. The reason is that driversgenerally do not want the gear change system to make arbitrary gearchanges “a bit here and a bit there”, which he/she may find nervous andarbitrary. “Same amount of time” in this context means that therespective times are of approximately equal length.

According to another preferred embodiment of the invention, changinggear at a intermediate gear step takes place at a higher engine speedthan at a preceding intermediate step, which means that the engine speedat gear changes increases for each intermediate step. With thisembodiment, the driver feels that the vehicle 1 is powerful and that it“applies itself” vigorously, e.g. on an upgrade, since it will run moreat the maximum power output of the engine 10 for each subsequentintermediate gear during the downshift. Provided that it does not exceedits maximum power output speed (often about 1800 rpm for trucks), ahigher engine speed will mean that the engine 10 delivers a higher poweroutput. The engine 10 thus becomes and is felt to be more powerful thehigher the engine speed at which the vehicle 1 is driven.

FIGS. 3A and 3B are together a flowchart of an exemplified embodiment ofa method according to the present invention. This flow is conceived asbeing evaluated continuously by the gear change system whenever thevehicle 1 is in power deficit, which means that the system assesseswhether a gear change should or should not be effected, starting fromstep F1 at predetermined intervals of time.

Step F1 checks whether a current engine speed is higher than a highestengine speed which the vehicle 1 has had since it has been in powerdeficit, i.e. α<0. If such is the case, this current engine speed issaved to become a first highest engine speed ω_(T) ^(i), for use as areference speed in the gear change process.

Step F2 chooses a suitable minimum engine speed ω_(Min) which is not tobe dropped below when the acceleration α is negative (i.e. when thevehicle 1 is in power defict), depending inter alia on how quickly thevehicle 1 loses speed, the road gradient derivative and the driving modebeing used by the vehicle 1. For example, a lower minimum engine speedω_(Min) is chosen if the road gradient derivative is decreasing, i.e.when the vehicle is approaching the crest of a hill. The minimum speedω_(Min) is preferably also chosen such that a current engine speed neverdrops below the maximum torque curve of the engine 10 to ensure that thelatter delivers sufficient power output throughout the downshiftprocess, and such as to avoid uncomfortable vibrations from the powertrain.

Step F3 chooses a suitable maximum engine speed ω_(Max) defined as aspeed not to be exceeded during the gear change process when α<0. Thismaximum speed ω_(Max) may for example be related to fuel consumptionand/or desired power output. For heavy vehicles 1, this maximum speedω_(Max) may for example be 1600 rpm to ensure that the fuel consumptionwill not be too high or 2100 rpm if maximum power output is prioritised.It will therefore be appreciated that said maximum speed ω_(Max) maydepend on the mode in which the vehicle is being driven.

Step F4 limits the engine speed ω_(T) ^(i) so that it assumes a valuewithin the range defined by the minimum speed ω_(Min) and the maximumspeed ω_(Max). Thus ω_(T) ^(i) may be set to the minimum speed ω_(Min)(ω_(T) ^(i)=ω_(Min)) if ω_(T) ^(i) assumes a value below the range, andto the maximum speed ω_(Max) (ω_(T) ^(i)=ω_(Max)) if ω_(T) ^(i) assumesa value above the range.

Step F5 seeks the second gear G2 by calculating to which of possibleavailable lower gears the vehicle 1 has to change in order to ensurethat the acceleration α is greater than or equal to nil in that gear,i.e. α≧0. The second gear G2 is calculated by the gear change systemchecking in which lower gear the vehicle's 1 driving power exceeds acalculated running resistance, i.e. the total force acting against thevehicle 1 in its direction of movement. For purely practical reasons,the second gear G2 may be calculated by the system calculating step bystep the vehicle's maximum driving force for gears lower than the firstgear G1, and choosing the first lower gear—if the calculation is donestep by step from highest to lowest gears—which has a maximum drivingforce as great as or greater than the calculated running resistance tothe vehicle 1.

Step F6 calculates thereafter the engine speed in the second gear G2 atwhich the driving power of the vehicle 1 exceeds or is equal to itsrunning resistance (α≧0), in order to be able to decide when a downshiftto the second gear G2 should be effected to ensure that the engine speedafter the downshift to the second gear G2 is close to the equilibriumspeed immediately after the downshift, the equilibrium speed being thatat which the acceleration of the vehicle 1 is substantially nil.

Step F7 calculates how long the vehicle 1 can run in each of theintermediate gears if there are a number of them between the first gearG1 and the second gear G2. How long the vehicle 1 can run in eachintermediate gear is compared with a gear-specific calibrated timevalue, i.e. a threshold value T_(i), for each intermediate gear. Thesecomparisons provide a basis for choosing the highest intermediate gearfrom among the permissible intermediate gears, i.e. the gears which havea calculated time value greater than their respective threshold valueT_(i). It should be noted that the gear chosen at step F7 is the secondgear G2 if no higher gear meets the requirements as above.

Moreover, step C1 checks at gear changes from the first gear G1 to seewhether the speed of the engine 10 has dropped more than a calibratedlimit value R from the highest engine speed which the vehicle 1 had whenit went into power deficit ω_(G1). The limit value R may vary dependingon driving mode, e.g. economy mode or power mode. Only when the enginespeed has dropped more than the calibrated limit value R (preferably1-100 rpm) is a downshift permissible. This limit value R is used in theprocess so that the vehicle 1 will not be felt to be nervous orunstable, e.g. when beginning to climb a hill, since otherwise theremight be a downshift as soon as the road gradient so allows. This checkat step C1 need not be done from intermediate gears, since the vehiclewill then already be on a hill, which means that the check at C1 is onlyrelevant from the first gear G1.

At step F8 no downshift takes place if the check at C1 shows that theengine speed has not dropped more than the limit value R. This is toprevent the system from changing gear when the running resistancetemporarily increases, e.g. on short hills, which would make the vehicle1 feel nervous.

Step C2 checks whether the current engine speed is close to the minimumspeed ω_(Min), and if such is the case an immediate downshift iseffected at step F9 to the gear chosen at step F7, since there is thenrisk of the engine dropping below the minimum speed ω_(Min).

If such is not the case, step F10 predicts the engine speed which thevehicle 1 will have after a downshift to the gear chosen at step F 7,i.e. ω_(T) ^(i+1), if the current engine speed at step C2 is not belowthe minimum speed ω_(Min).

Thereafter, step C3 checks whether the current engine speed ω_(T) ^(i+1)predicted at step F10 is less than ω_(T) ^(i)+p^(i), which is thehighest speed which the engine 10 has had since it went into powerdeficit, and a tolerance value p_(i) according to ω_(T) ^(i+1)=ω_(T)^(i)+p_(i), i=1, 2, 3 . . . , as described above. It should be notedthat according to this embodiment of the invention the first highestengine speed ω_(T) ¹ is the same as the highest speed in the first gearG1, i.e. ω_(T) ¹=ω_(G1) (see step F1). The highest engine speed in thefirst gear G1 will therefore affect the whole downshift process and thusserve as an input parameter for it.

Step F11 effects a downshift to the gear chosen at step F7 if the checkat step C3 shows that the current engine speed is within the range I.

If such is not the case, step F12 calculates the time for which thegearbox 20 can run in the gear chosen at step F7. In a practicalapplication, the gear change system calculates how long the vehicle 1can run in each intermediate gear from the first gear G1 to the secondgear G2, and chooses thereafter the highest of these intermediate gearswhich meets the requirement that its calculated time value be greaterthan its gear-specific calibrated threshold value T_(i).

When calculating how long the vehicle 1 can run in an intermediate gear,the system can use the minimum speed ω_(Min) which is a lowest enginespeed not to be dropped below by the vehicle 1, so if a current speeddrops to below, for example, ω_(Min)=1100 rpm, the system has to effecta downshift. It then calculates how long the vehicle 1 can run in theintermediate gear from the engine speed which the system is at afterchanging gear until the system reaches the minimum speed, i.e.ω_(Min)=1100 rpm. The system therefore calculates how long the vehicle 1will be able to run in the intermediate gear by working out how quicklythe engine 10 will lose speed in the intermediate gear on the basis ofknowing its minimum speed ω_(Min) and what its speed after changing gearwill be.

Finally, step C4 checks whether the time calculated at step F12 is aslong as the time for which the vehicle 1 has run in a current gear sincethe acceleration α became negative. Step F14 effects a downshift to thegear chosen at step F7 if the check at C4 shows that the answer isaffirmative. If the answer at step C4 is negative, no downshift takesplace at step F13.

FIG. 4 depicts an example of a downshift of a motor vehicle 1 accordingto the invention, in a diagram in which the x axis represents time andthey axis the speed of the engine 10 in rpm. At time t1 when travellingwith the first gear G1 engaged, the vehicle 1 embarks upon an upgrade,so a power deficit occurs and the vehicle's 1 acceleration α becomesnegative. The engine speed in the first gear G1 ω_(G1) is determined atthis time t1 and can be used as an input parameter during the downshiftaccording to an embodiment of the invention.

As the vehicle 1 is in power deficit, the engine speed decreases and afirst downshift takes place at time t2, causing the engine speed to goback up at t3. In this next gear too (first intermediate gear) a powerdeficit occurs and a further downshift needed to enable the vehicle toclimb the hill takes place at time t4. This is repeated in FIG. 4 inthat the engine speed goes back up t5, a power deficit occurs, and afurther downshift takes place at t6 and the engine speed goes back up tot7. Thus the downshift involves one or more intermediate gear stepsuntil the gearbox 20 reaches a power equilibrium gear, i.e. the secondgear G2, at time t7 in FIG. 4. In this second gear G2 (from time t8) theacceleration α is substantially nil, so the vehicle 1 can maintain itsspeed in this gear.

It should also be noted that the highest engine speed for eachintermediate gear is higher than or equal to the highest speed since thevehicle 1 went into power deficit, i.e. when the acceleration α becamenegative, as depicted at times t3, t5 and t7 in FIG. 4. This is done sothat the vehicle 1 will maintain its speed uphill as well as possible bybeing close to the maximum power output speed of the engine 10 in theintermediate gears. It also means that the vehicle 1 will be felt to beaggressive and powerful on, for example, long steep hills if it travelsat high speed, since the engine speed will rise at each downshift stepand thus come closer to the maximum engine speed which the vehicle canmaintain in the second gear G2. Better maintenance of vehicle speed atthe beginning of the climb also makes it possible to reduce the numberof downshift steps if the hill ends before the vehicle 1 reaches thesecond gear G2.

It should also be noted that the various calculation steps in the methodaccording to the invention take place in real time, as specialists willappreciate. They will also appreciate that a method for control of agearbox according to the present invention might also be implemented ina computer program which, when executed in a computer, causes thecomputer to effect the method. The computer program is contained in acomputer program product's computer-readable medium which takes the formof a suitable memory, e.g. ROM (read-only memory), PROM (programmableread-only memory), EPROM (erasable PROM), flash memory, EEPROM(electrically erasable PROM), hard disc unit, etc.

The present invention relates also to a system for control of a gearbox.The system comprises at least one control unit 110 intended to control agearbox 20 in a motor vehicle 1 and adapted to effecting a downshiftfrom a first gear G1 for which the acceleration α is negative to asecond gear G2 for which the acceleration α is positive or equal to nil.The downshift involves at least one intermediate gear step between thefirst gear G1 and the second gear G2, and an engine speed ω_(G1) in saidfirst gear (G1) is used as an input parameter when effecting saiddownshift.

A system described above may be also be modified according to thevarious embodiments of the above method. The present invention relatesalso to a motor vehicle 1, e.g. a truck or bus, which comprises at leastone system as above.

FIG. 5 depicts schematically a control unit 110 forming part of a systemaccording to the invention. The control unit 110 comprises a calculationunit 111 which may take the form of substantially any suitable type ofprocessor or microcomputer, e.g. a circuit for digital signal processing(digital signal processor, DSP) or a circuit with a predeterminedspecific function (application specific integrated circuit, ASIC). Thecalculation unit 111 is connected to a memory unit 112 which isincorporated in the control unit 110 and which provides the calculationunit 111 with, for example, the stored programme code and/or the storeddata which the calculation unit 111 needs in order to be able to performcalculations. The calculation unit 111 is also adapted to storingpartial or final results of calculations in the memory unit 112.

The control unit 110 is further provided with devices 113, 114, 115, 116for respectively receiving input signals and sending output signals.These input and output signals may comprise waveforms, pulses or otherattributes which the signal receiving devices 113, 116 can detect asinformation and which can be converted to signals processable by thecalculation unit 111. The calculation unit 111 is then provided withthese signals. The signal sending devices 114, 115 are adapted toconverting signals received from the calculation unit 111 in order tocreate, e.g. by modulating the signals, output signals which can betransmitted to other parts of the system for determination of downshiftand upshift points. One skilled in the art will appreciate that theaforesaid computer may take the form of the calculation unit 111 andthat the aforesaid memory may take the form of the memory unit 112.

Each of the connections to the devices for respectively receiving inputsignals and sending output signals may take the form of one or more fromamong the following: a cable, a data bus, e.g. a CAN (controller areanetwork) bus, a MOST (media orientated systems transport) bus or someother bus configuration, or a wireless connection. The connections 70,80, 90, 100 in FIG. 1 may also take the form of one or more of thesecables, buses or wireless connections.

Finally, the present invention is not limited to its embodimentsdescribed above, but relates to and comprises all embodiments within thescope of protection of the attached independent claims.

The invention claimed is:
 1. A method for control of a gearbox,installed in a motor vehicle, the method comprising effecting adownshift of the gearbox from a first gear for which acceleration α ofthe vehicle is negative to a second gear for which the acceleration α ispositive or substantially equal to nil, wherein the downshift involvesat least one intermediate gear step between the first gear and thesecond gear, using an engine speed ω_(G1) in the first gear as an inputparameter when effecting the downshift, such that the engine speedω_(G1) in the first gear is an engine speed which the vehicle assumeswhen the acceleration α of the vehicle becomes negative in the firstgear.
 2. A method according to claim 1, wherein the engine speed ω_(G1)in the first gear is a highest engine speed in the first gear.
 3. Amethod according to claim 1, wherein each intermediate gear is chosensuch that each intermediate gear step lasts a longer time than athreshold value.
 4. A method according to claim 3, further comprisingusing the threshold value as a parameter for determining a number ofgear steps involved in a gear change between at least one of the firstgear and an intermediate gear step, and between two consecutiveintermediate gear steps, and between an intermediate gear step and saidsecond gear.
 5. A method according to claim 3, further comprising usingthe threshold value as a parameter for determining the number ofintermediate gear steps between the first gear and the second gear.
 6. Amethod according to claim 3, wherein the threshold value is specific foreach intermediate gear step.
 7. A method according to claim 3, whereinthe threshold value is within a range of 1-15 seconds.
 8. A methodaccording to claim 3, wherein a current intermediate gear step lasts thesame amount of time as an immediately following intermediate gear step.9. A method according to claim 1, wherein an engine speed during gearchange at an intermediate gear step is higher than an engine speedduring gear change at a preceding intermediate gear step.
 10. A methodaccording to claim 1, wherein a gear change between at least one of thefirst gear and an intermediate gear step, and between two consecutiveintermediate gear steps, and between an intermediate gear step and thesecond gear comprises one or more gear steps.
 11. A method according toclaim 1, wherein a highest engine speed at each intermediate gear stepis as high as, or higher than, a highest engine speed at a precedingintermediate gear step.
 12. A method according to claim 11, wherein thehighest engine speed at each intermediate gear step assumes a valuewhich is p_(i) higher than said highest engine speed at a precedingintermediate gear step, p_(i) a parameter value.
 13. A method accordingto claim 12, in which a relationship between the highest engine speed ateach intermediate gear step and the highest engine speed at a precedingintermediate gear step is expressed by ω_(T) ^(i+1)=ω_(T) ^(i)+p_(i),where ω_(T) ^(i+1) is the highest engine speed at each intermediate gearstep and ω_(T) ^(i) is the highest engine speed at a precedingintermediate gear step.
 14. A method according to claim 1, wherein agear change between at least one of the first gear and an intermediategear step, or between two consecutive intermediate gear steps, orbetween an intermediate gear step and the second gear comprises one ormore gear steps.
 15. A computer program product comprising anon-transitory computer-readable storage medium and a computer programcontained in said computer-readable medium, the computer programcomprises program code, and when the program code is executed in acomputer, causes the computer to effect a method according to claim 1.16. A method according to claim 3, further comprising using thethreshold value as a parameter for determining a number of gear stepsinvolved in a gear change between at least one of the first gear or anintermediate gear step, or between two consecutive intermediate gearsteps, or between an intermediate gear step and said second gear.
 17. Asystem for control of a gearbox in a motor vehicle, the system comprisesat least one control unit to control the gearbox the control unit andthe system is effecting a downshift of the gearbox from a first gear forwhich an acceleration α of the vehicle is negative to a second gear forwhich the acceleration α is positive or equal to nil, wherein thedownshift involves at least one intermediate gear step between the firstgear and the second gear, using an engine speed ω_(G1) in the first gearas an input parameter when effecting said downshift, whereby the enginespeed ω_(G1) in the first gear is an engine speed which the vehicleassumes when the acceleration α of the vehicle becomes negative in thefirst gear.
 18. A motor vehicle which comprises at least one systemaccording to claim 17.